Conventionally, a disc brake is used as a brake device of an automobile, and a disc brake pad manufactured by fixing the friction material on a metallic base member is used as a friction member of the disc brake.
The friction material is classified into three types, i.e., a semi-metallic friction material containing, as a fiber base material, 30 weight % or more but less than 60 weight % of a steel fiber relative to the total amount of the friction material composition, a low steel friction material containing a steel fiber in a part of the fiber base material as well as less than 30 weight % of the steel fiber relative to the total amount of the friction material composition, and the NAO friction material containing no steel fiber and steel-based fiber such as a stainless steel fiber.
In the late years where the friction material causing less braking noise is in demand, the trend is to use the disc brake pad that utilizes the NAO friction material which mainly contains a binder, a fiber base material, a lubricant, an inorganic friction modifier, an organic friction modifier, pH adjuster, and a filler, and which contains no steel fiber and no steel-based fiber but contains a titanate.
For the NAO friction material for the disc brake pad, in order to secure the required performance, about 5-20 weight % of a copper component such as fibers and/or particles of copper and/or copper alloy in total relative to the total amount of the friction material composition, is added as a necessary component for the NAO friction material composition.
However, recently, the above-described friction material including the copper component, when braking, discharges the copper as an abrasion powder, and it is suggested that the discharged copper flows into a river, lake, and/or ocean and then the copper possibly contaminates a body of water.
Because of these backgrounds, for example, California State (CA) and Washington State (WA) of the United States of America passed a bill to prohibit the sales of the friction member using the friction material containing 5 weight % or more of the copper component relative to the total amount of the friction material composition and an act of assembling the subject friction material in a new car from the year of 2021 and to prohibit the sales of the friction member using the friction material containing 0.5 weight % or more of the copper component relative to the total amount of the friction material composition and an act of assembling the subject friction material in a new car either from 2023 or later or 2025 or later.
Then, a developmental study of the friction material that satisfies the laws and regulations of copper content restriction while securing a required performance for the friction material has been proceeding.
The Patent Document 1 discloses a disc brake pad using a NAO friction material composition that includes a binder, an organic filler, an inorganic filler and a fiber base, in which a copper content in the friction material composition is 5 mass % or less as a copper element, a metal fiber content other than a copper and copper alloy is 0.5 mass % or less, a titanate and a zirconium oxide with a particle diameter of 30 μm or less is included, the titanate content is 10-30 mass %, and the zirconium oxide with a particle diameter of over 30 μm is not included substantially.
The Patent Document 2 discloses a disc brake pad using a (NAO) friction material composition that includes a binder, an organic filler, an inorganic filler and a fiber base, in which a copper content in the friction material composition is 5 mass % or less as a copper element, a metal fiber content other than a copper and copper alloy is 0.5 mass % or less, a titanate and zinc powder are included, and the titanate content is 10-35 mass %.
As in the Patent Document 1 and the Patent Document 2, when relatively larger amount of the titanate such as the potassium titanate and lithium potassium titanate is added to the non-asbestos-organic (NAO) friction material, a transfer layer of the titanate is formed on a sliding surface of the disc rotor as a contacting member when braking the disc rotor with the friction material, and an adhesive friction occurs between the transfer layer and the titanate existing on the friction material surface, thereby obtaining the friction material that enables to secure the sufficient friction coefficient and effective braking performance even if the copper component is reduced.
However, the titanate is an expensive material for the friction material, and there is a problem of not being able to offer a low cost friction material when using the technology disclosed in the Patent Document 1 and the Patent Document 2.